[SPARK-695] In DAGScheduler's getPreferredLocs, track set of visited partitions.
Remove apparently incorrect comment describing NarrowDependency.

Each of a CartesianRDD's dependencies, for example, is a NarrowDependency, but child partitions of
these dependencies may depend on shared parent partitions.  For example, the cartesian product of
RDDs a and b containing partitions [ a_0 ] and [ b_0, b_1 ] respectively will be partitioned as
[ ( a_0, b_0 ), ( a_0, b_1 ) ].  Each child partition depends on parent partition a_0.
Clarify comment.
Remove unused variable.
Fix indentation.